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Marketable Investments - Summary of Maturity Periods of the Marketable Securities (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]    
Total   $ 81,013us-gaap_AvailableForSaleSecurities
Federal Agency and Corporate Obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
FY 2015 21,076us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= forr_FederalAgencyAndCorporateObligationsMember
 
FY 2016 16,109forr_AvailableForSaleSecuritiesDebtMaturitiesYearTwoFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= forr_FederalAgencyAndCorporateObligationsMember
 
FY 2017 17,700forr_AvailableForSaleSecuritiesDebtMaturitiesYearThreeFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= forr_FederalAgencyAndCorporateObligationsMember
 
Total $ 54,885us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= forr_FederalAgencyAndCorporateObligationsMember
$ 74,199us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= forr_FederalAgencyAndCorporateObligationsMember